Re: [PATCH net] ixgbe: napi_poll must return the work done

[Keller, Jacob E]

On Wed, 2016-06-15 at 09:34 -0700, Venkatesh Srinivas wrote:
> Reviewed-by: Venkatesh Srinivas 
> 
> The same bit of code appears in fm10k and i40e/i40evf. ixgb appears
> to
> correctly return work_done.
> 
> ixgbe_poll also appears to return an (minor) incorrect work_done in
> another case, BTW. It divides its
> budget between Rx rings associated with a vector. If any ring exceeds
> its share of the budget, ixgbe_poll
> claims to have consumed the full budget, even if a full budget of
> frames was not received in a single
> pass.
> 

So the correct return here would also be min between work_done and
budget, right? ie: we should still return the total work done instead
of the budget...?

Thanks,
Jake

> -- vs;

Re: [PATCH net] ixgbe: napi_poll must return the work done

[Keller, Jacob E]

On Wed, 2016-06-15 at 09:34 -0700, Venkatesh Srinivas wrote:
> On Wed, Jun 15, 2016 at 6:37 AM, Paolo Abeni 
> wrote:
> > 
> > Currently the function ixgbe_poll() returns 0 when it clean
> > completely
> > the rx rings, but this foul budget accounting in core code.
> > Fix this returning the actual work done, capped to weight - 1,
> > since
> > the core doesn't allow to return the full budget when the driver
> > modifies
> > the napi status
> > 
> > Signed-off-by: Paolo Abeni 
> > ---
> >  drivers/net/ethernet/intel/ixgbe/ixgbe_main.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> > b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> > index 088c47c..8bebd86 100644
> > --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> > +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> > @@ -2887,7 +2887,7 @@ int ixgbe_poll(struct napi_struct *napi, int
> > budget)
> > if (!test_bit(__IXGBE_DOWN, >state))
> > ixgbe_irq_enable_queues(adapter, BIT_ULL(q_vector-
> > >v_idx));
> > 
> > -   return 0;
> > +   return min(work_done, budget - 1);
> >  }
> > 
> >  /**
> Reviewed-by: Venkatesh Srinivas 
> 
> The same bit of code appears in fm10k and i40e/i40evf. ixgb appears
> to
> correctly return work_done.
> 
> ixgbe_poll also appears to return an (minor) incorrect work_done in
> another case, BTW. It divides its
> budget between Rx rings associated with a vector. If any ring exceeds
> its share of the budget, ixgbe_poll
> claims to have consumed the full budget, even if a full budget of
> frames was not received in a single
> pass.
> 
> -- vs;

For the record, I couldn't find any documentation on this in
Documentatino/networking, or as a function header. Where would be the
best place to document the expectations of the napi core? I'd like to
submit a patch so that future it will be easier to determine what a new
driver should do (without just blindly copying from other drivers as
has caused these so far).

Thanks,
Jake

Re: [PATCH net] ixgbe: napi_poll must return the work done

[Keller, Jacob E]

On Wed, 2016-06-15 at 09:34 -0700, Venkatesh Srinivas wrote:
> On Wed, Jun 15, 2016 at 6:37 AM, Paolo Abeni 
> wrote:
> > 
> > Currently the function ixgbe_poll() returns 0 when it clean
> > completely
> > the rx rings, but this foul budget accounting in core code.
> > Fix this returning the actual work done, capped to weight - 1,
> > since
> > the core doesn't allow to return the full budget when the driver
> > modifies
> > the napi status
> > 
> > Signed-off-by: Paolo Abeni 
> > ---
> >  drivers/net/ethernet/intel/ixgbe/ixgbe_main.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> > b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> > index 088c47c..8bebd86 100644
> > --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> > +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> > @@ -2887,7 +2887,7 @@ int ixgbe_poll(struct napi_struct *napi, int
> > budget)
> > if (!test_bit(__IXGBE_DOWN, >state))
> > ixgbe_irq_enable_queues(adapter, BIT_ULL(q_vector-
> > >v_idx));
> > 
> > -   return 0;
> > +   return min(work_done, budget - 1);
> >  }
> > 
> >  /**
> Reviewed-by: Venkatesh Srinivas 
> 
> The same bit of code appears in fm10k and i40e/i40evf. ixgb appears
> to
> correctly return work_done.
> 
> ixgbe_poll also appears to return an (minor) incorrect work_done in
> another case, BTW. It divides its
> budget between Rx rings associated with a vector. If any ring exceeds
> its share of the budget, ixgbe_poll
> claims to have consumed the full budget, even if a full budget of
> frames was not received in a single
> pass.
> 
> -- vs;

I can submit a patch for fm10k.

Thanks,
Jake

Re: [PATCH net] ixgbe: napi_poll must return the work done

[Venkatesh Srinivas]

On Wed, Jun 15, 2016 at 6:37 AM, Paolo Abeni  wrote:
> Currently the function ixgbe_poll() returns 0 when it clean completely
> the rx rings, but this foul budget accounting in core code.
> Fix this returning the actual work done, capped to weight - 1, since
> the core doesn't allow to return the full budget when the driver modifies
> the napi status
>
> Signed-off-by: Paolo Abeni 
> ---
>  drivers/net/ethernet/intel/ixgbe/ixgbe_main.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c 
> b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> index 088c47c..8bebd86 100644
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> @@ -2887,7 +2887,7 @@ int ixgbe_poll(struct napi_struct *napi, int budget)
> if (!test_bit(__IXGBE_DOWN, >state))
> ixgbe_irq_enable_queues(adapter, BIT_ULL(q_vector->v_idx));
>
> -   return 0;
> +   return min(work_done, budget - 1);
>  }
>
>  /**

Reviewed-by: Venkatesh Srinivas 

The same bit of code appears in fm10k and i40e/i40evf. ixgb appears to
correctly return work_done.

ixgbe_poll also appears to return an (minor) incorrect work_done in
another case, BTW. It divides its
budget between Rx rings associated with a vector. If any ring exceeds
its share of the budget, ixgbe_poll
claims to have consumed the full budget, even if a full budget of
frames was not received in a single
pass.

-- vs;

Re: [PATCH net] ixgbe: napi_poll must return the work done

[Paolo Abeni]

On Wed, 2016-06-15 at 08:20 -0700, Alexander Duyck wrote:
> On Wed, Jun 15, 2016 at 6:37 AM, Paolo Abeni  wrote:
> > Currently the function ixgbe_poll() returns 0 when it clean completely
> > the rx rings, but this foul budget accounting in core code.
> > Fix this returning the actual work done, capped to weight - 1, since
> > the core doesn't allow to return the full budget when the driver modifies
> > the napi status
> >
> > Signed-off-by: Paolo Abeni 
> 
> I think the origin of reporting 0 was actually compatibility with some
> NAPI code floating around from before the 2.6.24 kernel.
> 
> I'd be curious to know how much this is actually fouling things up.
> Can you point to any specific issues it was causing?  

I noticed this while instrumenting the napi poll loop for another
patch. 

It's not easy to reproduce the bugged scenario, several NICs receiving a
relevant amount of traffic on napi instances scheduled on the same
softirq are needed. 

If any/some of them has the buggy poll() method, the napi_poll() loop
may process (much) more than netdev_budget packets per invocation,
possibly delaying others softirq more than needed/expected. 

The maxium delay will be no matter what capped to a couple of jiffies,
due to the time-based loop end condition, so in the worst possible
scenario (most probably not a real thing), this adds a latency of 2
jiffies -  (~1.8ms on
recent h/w with HZ==1000).

> If you end up
> having to submit a v2 for any reason it might be useful if you can
> provide the additional details on what actual issue it was causing.
> 
> You might also want to look at the other Intel drivers, specifically
> ixgbevf and fm10k as I believe we have similar code in those drivers
> as well.

Thank you for the head-up. I need to get an hand on that h/w, first!

Paolo

> 
> Acked-by: Alexander Duyck 

Re: [PATCH net] ixgbe: napi_poll must return the work done

[Alexander Duyck]

On Wed, Jun 15, 2016 at 6:37 AM, Paolo Abeni  wrote:
> Currently the function ixgbe_poll() returns 0 when it clean completely
> the rx rings, but this foul budget accounting in core code.
> Fix this returning the actual work done, capped to weight - 1, since
> the core doesn't allow to return the full budget when the driver modifies
> the napi status
>
> Signed-off-by: Paolo Abeni 

I think the origin of reporting 0 was actually compatibility with some
NAPI code floating around from before the 2.6.24 kernel.

I'd be curious to know how much this is actually fouling things up.
Can you point to any specific issues it was causing?  If you end up
having to submit a v2 for any reason it might be useful if you can
provide the additional details on what actual issue it was causing.

You might also want to look at the other Intel drivers, specifically
ixgbevf and fm10k as I believe we have similar code in those drivers
as well.

Acked-by: Alexander Duyck 

[PATCH net] ixgbe: napi_poll must return the work done

[Paolo Abeni]

Currently the function ixgbe_poll() returns 0 when it clean completely
the rx rings, but this foul budget accounting in core code.
Fix this returning the actual work done, capped to weight - 1, since
the core doesn't allow to return the full budget when the driver modifies
the napi status

Signed-off-by: Paolo Abeni 
---
 drivers/net/ethernet/intel/ixgbe/ixgbe_main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c 
b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
index 088c47c..8bebd86 100644
--- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
+++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
@@ -2887,7 +2887,7 @@ int ixgbe_poll(struct napi_struct *napi, int budget)
if (!test_bit(__IXGBE_DOWN, >state))
ixgbe_irq_enable_queues(adapter, BIT_ULL(q_vector->v_idx));
 
-   return 0;
+   return min(work_done, budget - 1);
 }
 
 /**
-- 
1.8.3.1